This invention relates to the capture of energy from waves on the surface of a body of water and, particularly, to a power generating system using a taut moored wave energy absorber.
Various systems are known for capturing energy from waves passing along the surface of a body of water, particularly an ocean where quite large and powerful waves are consistently present.
Wave energy is lost by friction with the sea bottom as the sea becomes shallow (water depths of half a wavelength or less). This is most pronounced where wavelengths tend to be long. Furthermore, near shore area available for wave energy generating is often restricted due to harbor and leisure activities. Therefore, the preferred location for the present invention would be in water depths greater than 50 m in locations several miles from shore.
Wave Energy Converters (WECs) may be classified in different ways according to their operating principle and the ways in which they react with waves. In terms of practical application, only a very few types of device are presently, or in the recent past have been, in use or under test:
Point Absorbers
Point absorbers are usually axi-symmetric about a vertical axis, and by definition their dimensions are small with respect to the wavelength of the predominant wave. The devices usually operate in a vertical mode, often referred to as ‘heave’. Typically, a surface-piercing floating body rises and falls with the passing waves and reacts against the seabed through a rigid structure or a taut mooring. As such they are capable of absorbing energy arising from changes in the surface level rather than from forward motion of breaking seas. The theoretical limit for the energy that can be absorbed by a single isolated, heaving, axi-symmetrical point absorber has been shown to depend on the wavelength of the incident waves rather than the cross sectional area of the device. Thus the wavelength is a critically important criterion, resulting in the attraction of locating the point absorber devices well outside the region of breaking waves, and where they will be exposed to long wavelength ocean swell or ‘heave’.
Latching and Phase Control
Already disclosed is the idea to force the phase of the floating body speed to follow that of the waves load. The floating body must have a natural period significantly shorter than the wave periods. The heaving body is held at the top and/or bottom of its cycle by a hydraulically operated latching mechanism (functioning as a “parking-brake”), locking the heaving floating body to a long rod attached to the bottom of the wave channel. It is then released so that it resumes motion with speed in direction and in phase with the wave load. In this way greatly amplified motions and correspondingly larger power levels were achieved.
Variable Buoyancy Apparatus
A further development in self-reacting point absorbers incorporates a three-body point absorber comprising a surface floating body, a submerged variable buoyancy and an inertial mass. Such a device is known and described in the international application WO 99/28623.
The international application WO 0196738 discloses a wave energy conversion apparatus for harnessing energy from wave motion comprising two devices, each comprising a surface floating body and/or one submerged body below the surface with linkages between the two devices. Both devices are adapted to move relative to one another in response to passing waves or swell in the body of liquid. The relative movement between the two devices may be harnessed by the linkages between them. The connection between the surface floating body and the submerged body is sufficiently rigid to transmit tension and compression forces.
The apparatus may also include mooring systems that maintain the complete apparatus in a position that is consistent with design requirements and regulatory demands and not significantly inhibit its efficient operation. Further, the linked devices may also have elastic links or chains or shock absorbers or similar adaptations to absorb excessive relative movements in heave or surge or pitch that may be caused by breaking seas in storm conditions. Such elastic links or chains would normally be slack and may be weighted.
However, such an apparatus requires at least two bodies with a complex guiding system.                Other devices that have been developed:        
GB2267128 discloses an apparatus for harnessing tidal or wave power, comprising a base structure adapted to be anchored to the sea bed, a floating structure connected to the base structure, and variable volume bags trapped between relatively movable components of structures expanding or being compressed as a result of movements of the floating structure due to tidal or wave motion. Valve-controlled inlet and outlet means for the bags enable expansion of the bag which causes it to fill with sea water and compression of the bag which causes it to expel the water which thereafter is used to generate usable energy. One pressure chamber thus expels water as the floating structure rises and the other pressure chamber expels water as the floating structure falls.
US2004251692 discloses a wave-power unit for the production of electric power of a type comprising a floating body and an electric linear generator whose reciprocating alternator by connection means is connected to the floating body and whose stator is arranged to be anchored in the bed of a sea/lake.
A common problem with existing devices designed to recover significant amounts of energy from the sea is their complexity and cost. They are predominantly large structures, with rigid components, placed in a harsh environment. There is little use of well-proven components. Most devices proposed are very demanding in terms of engineering design, deployment and maintenance.
Since one of the main purposes of developing wave energy extraction devices is to reduce the cost of energy, the costs involved in maintaining and manufacturing the wave energy extraction devices is a very important factor in the practicability of the devices. Thus, there is a need in the art for a wave energy extraction device which is inexpensive to manufacture and maintain.
Another main problem remaining is how to design an apparatus that at the same time is the efficient in operation and is able to withstand harsh offshore conditions.